Conventionally, copying machines, facsimile apparatuses and the like have been commercially available as image processing apparatuses which are combinations of image input devices such as a scanner and image output devices such as a printer.
In such an image processing apparatus, particularly, a color copying machine, there is proposed a technique of adding add-on data such as an identification code (e.g., a machine number) to a copied image in order to prevent illicit use of copies, as disclosed in, e.g., Japanese Patent Laid-Open No. 6-111019. As add-on data, a color such as light yellow which is difficult for the human eye to recognize is used.